Although rotating polygon scanning mirrors are typically used in laser printers to provide a beam sweep or scan of the image of a modulated light source across a moving photosensitive medium, such as a rotating drum, there have also been prior art efforts to use a much less expensive flat mirror with a single reflective surface, such as a mirror oscillating in resonance, to provide the scanning beam. Unfortunately, these prior art efforts of using a scanning or oscillating mirror have required a compromise in performance in that only one direction of the resonant beam sweep could be used to print an image line at a right angle on a page. For example, to generate image lines that are at a right angle to a moving photosensitive medium, the scanning mirror generating the beam sweep is typically mounted at a slight angle to compensate for the movement of the photosensitive medium. It will be appreciated that the photosensitive medium typically moves at a right angle with respect to the beam sweep (such as a rotating drum). Unfortunately, if the mirror is mounted at a slight angle to compensate for medium movement during the forward beam sweep, the return beam sweeps will traverse a trajectory on the moving photosensitive drum which will be at an angle which is unacceptable with the first printed image line since the effect of the moving medium and the angle mounting of the mirror will now be additive rather than subtractive. Consequently, unlike the present invention, when such a single reflecting surface resonant mirror was used with these prior art efforts, it was necessary to interrupt the modulation of the reflected light beam and wait for the mirror to complete the return sweep or cycle and then again start scanning in the original direction. This requirement of only using one of the sweep directions of the mirror of course reduces the print speed and requires expensive and sophisticated synchronization between the mirror and the rotating drum.
The assignee of the present invention has recently developed a dual axis mirror with a single reflection surface described in U.S. patent application Ser. No. 10/384,861 filed Mar. 10, 2003, entitled “Laser Printer Apparatus Using a Pivoting Scanning Mirror”. This dual axis mirror uses a first set of torsional hinges for providing oscillating beam sweep such as a resonant beam sweep and a second set of torsional hinges that selectively moves the oscillating beam sweep in a direction orthogonal to the oscillating or resonant beam sweep. By dynamically controlling the orthogonal position of the beam sweep to compensate for movement of the photosensitive medium, both directions of the resonant beam sweep may be used to print parallel image lines. Alternately, two single axis mirrors can be arranged such that one mirror provides the resonant beam sweep and the other mirror controls the orthogonal position of the beam sweep to allow both directions of the resonant beam sweep to be used for printing.
It will also be appreciated by those skilled in the art that in addition to laser printing, control of the orthogonal (vertical) position of the oscillating or resonant scan allows a single surface or flat oscillating mirror to be used to provide a full frame of raster scans suitable for use on projection displays including micro projection displays such as cell phones, Personal Digital Assistants (PDA's), notebook computers and heads-up displays. However, if such displays are to be commercially acceptable, they must be small, low cost, robust enough to withstand greater than 1000 G's of shock, and stable over the operating temperature normally experienced by hand-held products.
Consequently, it will be appreciated that the high frequency scanning mirror is a key component to the success of such products. Further, since many of the applications for such mirror projection displays are battery powered, all of the components (including the scanning mirror) must be energy efficient.
Texas Instruments presently manufactures a two axis analog mirror MEMS device fabricated out of a single piece of material (such as silicon, for example) typically having a thickness of about 100–115 microns using semiconductor manufacturing processes. The layout consists of a mirror having dimensions on the order of a few millimeters supported on a gimbals frame by two silicon torsional hinges. The gimbals frame is supported by another set of torsional hinges, which extend from the gimbals frame to a support frame or alternately the hinges may extend from the gimbals frame to a pair of hinge anchors. This Texas Instruments manufactured mirror with two orthogonal axes is particularly suitable for use with laser printers and/or projection displays. The reflective surface of the mirror may have any suitable perimeter shape such as oval, rectangular, square or other.
Similar single axis mirror devices may be fabricated by eliminating the gimbals frame altogether and extending the single pair of torsional hinges of the mirror directly to the support frame or support anchors. Two single axis mirrors rather than one dual axis mirror may be used to generate the beam scan but may require more space. Other suitable designs of single axis mirrors may also be used.
One presently used technique to oscillate the mirror about a first axis is to provide electromagnetic coil on each side of the mirror and then driving the coils with an alternating signal at the desired sweep frequency. The same technique is also used to move the sweep of the beam orthogonal to maintain a parallel raster scan. The present invention, however, discloses improved techniques for generating a resonant beam sweep.